An encryption technique called “the digital signature” is available as a method of detecting a false alteration of digital data. In this technique, the genuineness of digital data is determined by collating a hash. Generally, the hash is collated by producing a hash from the current data and comparing the particular hash with the original hash written in the header portion of the data.
This approach effectively functions in a completely closed system (a system other than a general-purpose system, using a special data format and having no fixed users), but cannot meet the requirement of an open system (a general-purpose system using an ordinary data format and assuming a multiplicity of unspecified users). This is by reason of the fact that once the file format is changed, the approach becomes inapplicable any longer.
Another approach called “the electronic watermark” is available. This concerns a method in which data not related to digital contents are buried in the digital contents or in which the data buried are extracted and collated. The “electronic watermark” requires such a structure that the buried data are not easily erased by the process of editing, compression, transmission or conversion of the contents and that the buried data cannot be easily falsely altered or overwritten with false information. In this method, the copyright information or the like, once buried, can be extracted even from falsely altered data, and therefore the originality of the data can be substantiated. Also, the falsely altered position can be detected by comparing the falsely altered data with the substantiated original data.
In this method, however, data are required to be buried in advance, and therefore a device for burying the data is required. Also, the buried data, which can be readily extracted as long as a burial method is known, has a low durability. Further, the data burial unavoidably deteriorates the data quality.
A digital image data acquisition device including an analog-to-digital converter, on the other hand, has a noise characteristic inherent to the analog-to-digital (A/D) conversion process.
The output signal of the CCD providing a photoelectric conversion device, for example, is structurally known to contain noises typically including what is called a lead-out noise constituting the total noises generated in a CCD element, an analog circuit of a control system and an A/D converter at the time of reading the charge of the CCD element and a dark charge noise due to the dark current generated in a well under the photoelectric surface of the CCD.
FIG. 1 is a diagram for explaining the noises contained in the output of a CCD element. FIG. 1(A) shows a digital image picked up with a lens cap attached to a digital camera which is frequency-converted using the two-dimensional FFT (Fast Fourier Transform), and FIG. 1(B) a solidly black digital image (O in digital value) produced using a digital image editing program and frequency-converted by the two-dimensional FFT. FIG. 1 indicates that the output data of the CCD element contains the noises in composite fashion.
The digital image data acquisition device including the A/D converter, on the other hand, has a characteristic inherent to the pixel value of the digital image data in the A/D conversion process.